Sucrose fatty acid esters have been the subject of intensive investigations for decades, since they can be prepared from renewable, inexhaustible natural substances, such as sugars and fats. Both substance groups are employed as valuable, mild, physiologically acceptable and biodegradable additives in cosmetics, pharmacy, in foodstuffs, in animal feeds and as agrochemicals, for example for keeping fruit fresh.
Sucrose glycerides represent a particular form of the supply of sucrose fatty acid esters. In general, they are mixtures of sucrose esters and glycerides. According to European Standard E474, sucrose glycerides which are permitted for the foodstuffs sector must contain at least 40% of sucrose esters and at least 40% of glycerides.
The prior art discloses a number of processes for the preparation of sucrose esters. Thus the reaction of fatty acid methyl esters with sucrose in the presence of basic catalysts in various solvents, such as dimethylformamide or dimethyl sulfoxide as a solvent, is described, see, for example DE-C-10 52 388 or DE-C-12 62 988.
A great disadvantage of this process is that the solvents dimethylformamide (DMF) or dimethyl sulfoxide (DMSO) have to be removed from the reaction mixture without residue after the transesterification, in particular if the products are to be employed in the foodstuffs sector.
In principle, sucrose glycerides can be prepared by means of the transesterification of natural fats or vegetable oils with sucrose. An economical preparation of sucrose glycerides by means of transesterification on an industrial scale, however, is hardly realizable, since the resulting glycerol can only be removed with difficulty because of its high boiling point. Moreover, the thermal instability and the relatively limited solubility of sucrose in organic solvents causes considerable problems. The solvents, for example DMF, DMSO, N-methylpyrrolidone (MNP) or pyridine, must therefore be added in a large excess and yet removed without residue after the reaction. Thus DE-C-11 93 026 describes a process for the preparation of poly fatty acid esters of nonreducing oligosaccharides by transesterifying a nonreducing oligosaccharide with fatty acid esters of nonsugar alcohols, the reaction being carried out in the presence of a transesterification catalyst in the presence of pyridine as a reaction medium.
Similarly, U.S. Pat. No. 3,349,081 describes a process for the preparation of sucrose esters by transesterification of sucrose with natural triglycerides in dimethylformamide, followed by stripping off the reaction solvent and treating the residue with an aqueous/butanolic sodium chloride solution and evaporating the separated butanol layer to dryness.
Furthermore, the preparation of sucrose glycerides from tallow or palm kernel oil and sucrose in dimethylformamide is known from L. Bobichon, ACS Syn. Ser (1977), 41, Sucrochemistry Sympo. 1976, pp. 115 to 120. The products, CELINOLS.RTM., contain 39 to 42% of sucrose esters, 50 to 55% of glycerides and 50 to 100 ppm of DMF and are recommended as animal feed additives.
Recently, attempts have been made to avoid the disadvantages of the prior art caused by the use of solvents. GB-A-1 399 053 discloses a solvent-free transesterification process for the preparation of sucrose esters. According to this process, the transesterification of fatty acid glycerides with sucrose is carried out at 110 to 140.degree. C., in particular under the action of potassium carbonate in an amount from 5 to 12%, based on the total weight of the reaction mixture. The products obtained by this process are very dark, in particular brown, waxy materials. Moreover, the total yield of mono- and disucrose esters is clearly below 30%. This process yields a complex reaction mixture which to a high extent contains nonreacted starting materials.
GB-A-1 499 989 likewise describes the reaction of solid sucrose with an alkyl ester of a fatty acid having 1 to 6 carbon atoms in the alcohol unit and at least 8 carbon atoms in the fatty acid unit, the reaction being carried out in the presence of basic transesterification catalysts at a temperature of 110 to 140.degree. C. under atmospheric pressure in the absence of solvents. Here too, as can be seen from the working examples, potassium carbonate is particularly preferably employed as a catalyst. Despite the high amount of basic catalyst used and very long reaction time of 6 to 23 hours, the two abovementioned processes give very low yields of sucrose esters. Moreover, the reaction mixtures contain, to a considerable extent, unreacted alkyl esters, glycerides, soaps and sucrose. According to the product composition described therein, reaction mixtures of this type can hardly be suitable for foodstuff uses or even fulfill the requirements of the EC standard E474 for foodstuff emulsifiers.
FR-A-2 463 152 discloses a further process for the preparation of sucrose glycerides, in which in the first reaction stage a limited alcoholysis of the triglyceride with alcohol takes place at 80 to 180.degree. C. with addition of the basic catalyst potassium carbonate. In the second reaction stage, the mixture of alkyl ester, triglyceride and soap is transesterified with sucrose with fresh addition of potassium carbonate. The process thus comprises two reaction steps, necessitates the use of high amounts of catalyst and soaps and at the same time, however, yields a reaction mixture of sucrose esters (40 to 49%), glycerides and high amounts of undesired, unreacted alkyl esters (2 to 20%) as well as soap and sucrose.
EP-A-0 404 226 discloses a process for the purification of product-containing esters of nonreducing sugars and one or more fatty acids. In this case, the crude esterification product is subjected to an extraction with supercritical carbon dioxide in a complicated process.
DE-A-41 31 505 relates to a process for the work-up of the reaction mixture obtained in the solvent-free preparation of sucrose fatty acid esters by transesterification of sucrose with fatty acid alkyl esters, in particular fatty acid methyl esters, in the presence of a basic transesterification catalyst. In order to improve the economy of the process with identical or even improved quality of the final product, it is proposed that unreacted sucrose is filtered off at a temperature between the melting points of the sucrose employed and of the sucrose ester prepared and then unreacted alkyl esters are distilled off from the reaction mixture.